1. Field of the Invention
The present invention relates to an ion selective electrode of the flowthrough type for use in electrochemical measurement of ion species contained in a biological fluid.
2. Description of the Related Art
There are colorimetric method, flame method, atomic absorption method, coulometric-titration method, and electrode method, used as a measuring method of the electrolyte concentration in samples, such as a blood serum, plasma, and urine. Of these methods, the electrode method has come to be used broadly in recent sears.
Especially, the electrode method is mainly used for concentration measurement of electrolytes such as sodium ion, potassium ion, and chlorine ion, in a clinical inspection field.
The electrode method is a method of electrochemically detecting the reaction of an ion and a sensing substance using an ion-sensing membrane which confine the sensing material sensitive to an ion to be detected. A common method of detecting the electric signal from the sensing membrane is a detection with an Ag/AgCl electrode which is an internal electrode through an electrolyte solution. The method of producing the ion selective electrode used for the electrode method is that the ion-sensing membrane is beforehand made by methods such as casting and then a fragment of the membrane is stuck on an electrode main part. Moreover, when measuring electrolyte concentration in a sample in a clinical inspection, usually two or more ions, such as sodium, potassium, and chlorine, are simultaneously measured. For this reason, the ion selective electrode used in order to measure these ions is used as a complex electrode by combination, for example, unifying or glueing several ion-selective electrodes using packings, such as an O ring.
Since an adhesion area for sticking the ion-sensing membrane is needed for an electrode main part according to the above-mentioned manufacturing method, in order to stick the ion-sensing membrane firmly, an electrode main part had to be large. Adhesion would be difficult if an adhesion side is small. Therefore, there is a limit in miniaturization of an electrode main part in such a method.
On the other hand, a CWE system (which an ion-sensing membrane is) stuck on an Ag/AgCl electrode is also put to practical use. According to this method, since an electrolyte solution is not necessary, it is possible to miniaturize an electrode main part. However, the ion-sensing membrane needs to be formed directly the Ag/AgCl electrode surface by applying material which forms the ion-sensing membrane. In this case, the surface of the ion-sensing membrane is uneven, therefore even if the surface of an ion-sensing membrane is washed with calibration liquid after measurement in this case, a sample tends to remain on the surface of the ion-sensing membrane.
Furthermore, according to the method of combining two or more ion selective electrodes, the path in an electrode through which a sample flows becomes long. Moreover, since the surface the junction side of the path in each electrode is not smooth and continuous, the sample often remains in the portion and an exact measurement result cannot be obtained.